One of the byproducts of combustion systems such as power plant boilers is exhaust gas, commonly known as flue gas. This gas may contain components which are harmful to the environment, such as oxides of nitrogen (NOx). The production of NOx can occur when fuels are combusted, such as in steam generators, steam boilers, refinery heaters, turbines, etc. Exemplary fuels include coal, oil, natural gas, waste product such as municipal solid waste, petroleum coke, and other carbon-based materials. It is beneficial to the environment to control the levels of NOx released into the atmosphere by burning such fuels.
Fired steam generators, e.g., boilers, produce nitrogen oxides during the combustion process. In order to meet air quality standards, nitrogen oxides are one of the groups of emissions which are controlled. One common approach used to reduce nitrogen oxides in the flue gas is through the use of a selective catalytic reduction process, which converts nitrogen oxides to nitrogen and water through the catalytic reaction of ammonia and platinum within a certain heat range. At lower boiler loads, when the gas temperature leaving the boiler is below the minimum operating temperature of the catalyst, unconverted ammonia will react with the water vapor and sulfur trioxides present in the flue gas to form ammonium bisulfate and ammonium sulfate. Ammonium bisulfate is a sticky and corrosive salt that tends to precipitate out on the catalyst plate causing pluggage and de-activation of the catalyst. To counter this, it is necessary to raise the gas temperature leaving the boiler above the minimum operating temperature of the catalyst in order that the NOx reduction reaction can be sustained to boiler loads down to approximately 30% of full boiler load.
Heat exchangers are devices used to exchange or transfer heat. In many steam generators or boilers, a heat exchanger, e.g., economizer, is used to transfer heat from the flue gas to the water feeding the steam generator or boiler. This approach saves energy by preheating the fluid, i.e., water, supplied or fed to the steam generator or boiler. By transferring the heat from the flue gas to the feed fluid, which is referred to as feedwater, the temperature of the flue gas is reduced. When the gas temperature leaving the boiler is below the minimum operating temperature of the catalyst, the downstream selective catalytic reducer's (SCR's) ability to operate properly to remove NOx from the flue gas is affected as described above.
In a known system that attempts to address the aforementioned problems, an economizer bypass system is employed wherein all or part of the feed water bypasses the entire economizer to raise the temperature of the exiting flue gas. However, such systems have many limitations and disadvantages including for example requiring: (1) additional safety valves because the output of a heated section may be closed off and (2) mixing equipment to prevent steam from leaving the economizer and going to the furnace circuits. Moreover, such systems can cause thermal shock and premature failure of the tubes since hot water flow through the economizer may be stopped altogether during some period of time with the economizer tubes then being subject to stresses as cold water is introduced into the dry hot economizer causing sudden temperature changes and/or flashing.
In view of the above discussion it should be appreciated that there is a need for new and improved ways of controlling the flue gas stream temperatures of steam generators or boilers in order to reduce the level of NOx entering the atmosphere via the exiting flue gas. While known attempts to address these needs have obtained some level of success, there remains a need for new and improved methods and systems for controlling flue gas temperatures. It is desirable that one or more new methods and apparatus be developed which work well particularly during low load operation of steam generators or boilers without risk of flashing or thermal shock and without the need for expensive safety values or mixing systems at the fluid outlet of a heat exchanger.